1. Field of the Invention
This invention relates to a hydraulic booster for vehicle which assists the vehicle driver with the operation of the master cylinder.
2. Description of the Prior Art
Japanese Patent Application No. 217892/1988 (Japanese Patent Opening Gazzette No. 68258/1990) discloses a hydraulic booster in which a second valve apparatus is arranged at a reaction chamber formed adjacent to a top end of an input member in order to reduce the initial drive force required to develop master cylinder pressure. The second valve apparatus prevents pressure-fluid from flowing into the reaction chamber until the pressure of fluid in the pressure-servo chamber becomes higher than a predetermined pressure. Thus, a force applied to the input member before the master cylinder pressure starts to rise, can be smaller. When the second valve apparatus is opened, there occurs the so-called "Jump-up phenomenon" in which the master cylinder pressure steeply rises up to a certain pressure.
When the pressure of the servo-chamber is lowered to relieve the braking action, the pressurized fluid from the servo-chamber is returned into the fluid pressure-releasing source arranged at the downstream side of the second valve apparatus. Accordingly, the flow amount of the pressurized fluid is increased, since the pressurized fluid is discharged also from the reaction chamber. Abnormal sound is made from the second valve apparatus. Further, since the fluid amount is much, there is the disadvantage that the brake-relieving action lags.